High-Efficiency\nInterdigitated Electrode-Based Droplet\nMerger for Enabling Error-Free Droplet Microfluidic Systems

Merging two droplets into a droplet to add and mix two\ncontents\nis one of the common droplet microfluidic functions with droplet generation\nand sorting, performing broad ranges of biological and chemical assays\nin droplets. However, traditional droplet-merging techniques often\nencounter unsynchronized droplets, causing overmerging or mis-merging,\nand unwanted merging outside of the desired zone. This is more severe\nwhen the incoming droplets to be merged are polydisperse in their\nsizes, often observed in assays that require long-term incubation,\nelevated-temperature, and/or multiple droplet processing steps. Here,\nwe developed an interdigitated electrode (IDE)-based droplet merger\nconsisting of a droplet autosynchronizing channel and a merging channel.\nThe autosynchronizing channel provides >95% merging efficiency\neven\nwhen 20% polydispersity in the droplet size exists. The highly localized\nand enhanced dielectrophoretic force generated by the IDEs on the\nchannel bottom allows droplet merging at an extremely low voltage\n(4.5 V) and only locally at the IDE region. A systematic evaluation\nof how various design and operation parameters of the IDE merger,\nsuch as IDE finger dimensions, dielectric coating layer thickness,\ndroplet size, and droplet flow speed impact the performance was conducted.\nThe optimized device showed consistent performance even when operating\nfor up to 100 h consecutively at high throughput (100 droplets/s).\nThe presented technology has been integrated into a droplet microfluidics\nworkflow to test the lytic activities of bacteriophage on bacterial\nhost cells with 100% merging efficiency. We expect this function to\nbe integrated into droplet microfluidic systems performing broad ranges\nof high-throughput chemical and biological assays.